Introduction
Flyash, a by-product from thermal power stations, is a
pozzolana and is being used in concrete construction. The
chemical admixture superplasticizer is also being used in concrete
along with ordinary Portland cement (OPC) and flyash. But the
quality of flyash should conform to IS 3812 (Part 1)1.
The following typical properties of flyash must be satisfied:
Fineness (Blaine’s): Minimum 320 m2/kg.,
Lime Reactivity: Maximum 4.5 N/ mm2,
Loss on ignition: Maximum 5.0%
Fortunately, these days, the quality of cement and the quality
of superplasticizer is good, and therefore, concrete construction
has also improved. But a proper mix proportioning method had
to be followed, so that the site engineers can adapt it easily, in
order to obtain the desired workability and compressive strength
of concrete. In this paper, an attempt has been made to develop
a concrete mix design procedure for normal grade of concrete,
using flyash and superplasticizer.Concrete-Making Materials
(OPC+flyash) or Portland pozzolana cement (PPC)
satisfying the requirements of Indian Standards can be used.
The superplasticizer commonly being used for normal grades
of concrete say up to M60 grade, is napthalene-based. Coarse
aggregates are generally crushed rock aggregates, and the
fine aggregate is generally riverbed sand of grading zones I,
II, III or IV as per Table 4 of IS 3832. IS 383 gives a cautionary
note that “the fine aggregate conforming to grading zone IV
should not be used in reinforced concrete unless tests have been
made to ascertain the suitability of proposed mix proportions”.
Sometimes, riverbed sand is not available, and in that case,
crushed stone fine aggregate can be used in concrete, and IS
383 specifies maximum fines content passing 150-micron IS
sieve is 20%. For smooth gravel or partly crushed gravel coarse
aggregate, the mixing water requirement of concrete will be
less than that for concrete using the crushed rock aggregate.
IS 102623 stipulates reduction of mixing water by 25kg/ m3
of concrete for rounded gravel aggregate, and by 10kg/ m3 of
concrete for sub-angular coarse aggregates.
Available Flyash Concrete Mix Design Procedures
1. Stipulation in the Indian Standard
There are no flyash concrete mix design procedures in the
Indian Standard IS 10262. The guidelines however, refer to the
standard IS 3812 (Part 1) for flyash, and an illustrative example
of mix proportioning for M40 grade concrete using 30% flyash
and 2% superplasticizer has been outlined. The illustrative
example suggests the following:
a) “Decide the percentage of flyash to be used based on project
requirement and quality of materials,
b) In certain situations, increase in cementitious material
content may be warranted. The decision to increase in
cementitious material content and its percentage may be
based on experience and trial”.
The illustrative example considers a 10% increase in
cementitious material content than the cement content of
corresponding OPC concrete.2. The British Flyash Concrete Mix Design Method
The British flyash concrete mix design method considers
improved workability because of flyash and therefore, a
reduction in the water content of the concrete mix. Typical
water-reduction values for a workability range of 30-60mm
slump suggested are: 10kg/ m3 of concrete for the use of 20%
flyash, 20kg/ m3 of concrete for the use of 30% flyash etc.
To offset the reduction in early strength, the mass of the
cementitious material (compared to the mass of cement in the
OPC concrete mix) has been increased. The Department of
Environment4 of England States that “flyash is used to replace
some of the cement in the mix, but in order to obtain concrete
having the same strength at 28 days, the combined weight of
the cement plus flyash needs to be greater than that of OPC in
a OPC concrete mix. BS 5328 Part15 specifies the increase in
the combined mass of OPC plus flyash by about 10% by mass.
The British method stipulates the quality of flyash with its loss
on ignition restricted to 7% and the residue on the 45 micron
sieve restricted to 12.5%. The British method further considers
Smith’s6 ‘Cementing efficiency factor (k),’ where KF is the mass
of OPC equivalent to a mass F of flyash, the value of K considered
as 0.30. To design a flyash concrete mix with a specified 28-day
compressive strength, the w (c+0.3F) ratio shall be used as free w/c
ratio in the relationship between w/c ratio Vs compressive strength
of concrete. The British method provides relationships between
water-cement ratio and compressive strength of concrete, charts
for finding the wet density of concrete for crushed and uncrushed
aggregates, proportion of fine aggregate depending on maximum
size of aggregate (MSA), grading of fine aggregate and the required
workability of concrete in terms of slump or V-B time.3. The ACI High-Strength Flyash Concrete Mix Design Method
The American high-strength flyash concrete mix
proportioning method7 as reported by ACI Committee 211
provides guidelines for selecting proportions for high-strength
flyash concrete mixes using high-range water-reducing
admixtures, the 28-day cylinder compressive strength of such
concrete being 42-84 N/ mm2. The method recommends
15-25% flyash replacement for class F flyash, and 20-35%
replacement for class C flyash (the flyash produced from lignite
coal and containing lime more than 10%). The preferred flyash
for use in high-strength concrete should have ‘loss on ignition’
value not greater than 3%, and should have high fineness. The
suggested MSA is 20-25mm for concrete compressive strength
less than 63 MPa and 10-12mm for concrete compressive
strength more than 63 MPa. The fineness modulus of sand
suggested is 2.5-3.2.
The American method is a little complicated, as it considers
the volume of oven-dry rodded aggregates, calculates void
content in fine aggregate, and requires trial mixture to be
conducted for the basic concrete mixture (without any flyash),
for obtaining 25-50mm slump of concrete, before adding any
superplasticizer. They state that trial is needed to ensure an
adequate amount of watet for mixing, and the superplasticizer
to be effective. The American method provides specific tables
for obtaining (a) fractional volume of even-dry rodded coarse
aggregate for different MSA, and for sand fineness modulus of
2.5 to 3.2, and (b) mixing water content for the desired slump
of concrete for different MSA. The table includes entrapped air
content to be considered for different MSA in concrete. Also
the method provides maximum water / (cement + flyash) i.e
W/(C+F) ratios for concrete with and without any high-range
water-reducing admixtures, for different MSA and different field
strength of concrete to be achieved at 28 and 56 days.Comments on the Mix Design Methods
The Indian Standard IS 10262 does not provide much detail on
the flyash concrete mix design procedure. The illustrative example
on M40 grade concrete using 30% flyash and 2% superplasticizer
considers increase in cementitious material content of 10% than
the cement content of corresponding OPC concrete.
The British mix design procedure considers improved
workability of concrete because of the inclusion of flyash in
concrete and hence provides reduced water content of concrete
for inclusion of different percentage of flyash in concrete. This
may not be true for all flyashes. Also, in our cement factories,
coarse flyash is ground and blended with OPC and in this process,
the requirement of water content of concrete mix using PPC
becomes more than that of concrete with (OPC + good quality
flyash). The British method considers flyash of much finer variety,
i.e. particles retained on 45 micron sieve restricted to 12.5%,
whereas in IS 3812 (Part 1), it has been restricted to 34%. With
12.5% residue on 45 micron sieve, the Blaine’s fineness of flyash
will be very high.
The ACI high-strength concrete mix design method also
considers very good quality of flyash, where the ‘loss on
ignition’ value has been restricted to 3%, and the flyash is of
high-fineness. The method considers the cylinder compressive
strength of concrete, and calculates void content in aggregates
based on oven-dry rodded unit weight of aggregates.
As such, the British and American methods of flyash concrete
mix design are based on specific tables and charts, which cannot be
followed by us, for the quality of Indian concrete-making materials.
The proposed flyash concrete mix proportioning
procedure, using a Superplasticizer.
Estimation of W/(C+F) Ratio
The target 28-day compressive strength of concrete (fck/)
has to be calculated first for the grade of concrete, from the
following formula:
fck/ = fck + 1.65 x S.D., where
fck = characteristic compressive strength of concrete at 28
days in N/ mm2, and
S.D. = Standard deviation in N/ mm2.
It is better to use the actual standard deviation value obtained
from the sites of construction. For a ‘good’ control, the standard
deviation value of 4 N/ mm2 can be assumed irrespective of the
grade of concrete4. Using an established correlation between W/
(C+F) ratio and the 28-day compressive strength of concrete,
the W/(C+F) ratio is to be estimated for the target strength.
Typical correlation is shown in fig. 1.

Estmation of Water Content of Concrete
The water requirement of the concrete mix is to be fixed, for
the desired workability of concrete. Generally, a naphthalenebased
superplasticizer is being used, either to reduce the water
content or to increase the workability of concrete. About 0.80
to 1.2% by weight of (C+F) is being used. For a low slump
concrete, say 25-30mm slump, about 20% mixing water can
be reduced using this type of superplasticizer. For increasing
the slump from 50-60mm to about 100-120mm, this much
percentage of superplasticizer, i.e. 0.8-1.2% is required to be
added to a concrete mix having water content of say 160-
180kg/ m3 of concrete. The water requirement of concrete
mix is primarily dependent on the MSA used in the concrete mix.
The mixing water content of concrete is also dependent on the
quality of flyash and its quantity in concrete. Typically, using a
good quality flyash (the Blaine’s fineness of about 320 m2/kg
and a lime reactivity value of about 4.5 N/ mm2), using 20mm
MSA in concrete, the mixing water requirement is about 170kg/
m3 of concrete, for a normal 50-60mm slump of concrete. For
40mm MSA in concrete, the water requirement will be less, say
about 150kg/ m3 of concrete. This water content of concrete
is for crushed rock saturated surface dry (S.S.D.) aggregates.
For gravel aggregates or partly crushed gravel aggregates, the
mixing water requirement will be lower, typically 160kg/ m3
and 140kg/ m3 of concrete for 20mm MSA and 40mm MSA is
concrete, respectively. The water requirement of concrete for a
desired workability of concrete will also depend on the efficiency
of the superplasticizer used.The Quantity of Flyash in Concrete
The percentage of flyash in a concrete mix should be based
on project requirements. For example, for hydroelectric projects,
IS 4568 stipulates at least 25% flyash in concrete, in order to
combat the probable alkali-silica reaction in concrete. For mass
concrete construction, even 40% flyash has been used in the raft
foundation concrete of M50 grade, for the highest tower of the
world, Burj Khalifa9. The quantity of flyash in concrete also will
depend on the quality of OPC and the quality of flyash. With OPC
53-grade, more quantity of flyash (say 35 or 40%) can be used
in concrete. If the quality of flyash is very good, say its fineness is
more than 400 m2/kg and the lime reactivity value is more than
5N/ mm2, with OPC 43-grade, 35% flyash can safely be used.The Air Content of Concrete
The air content of concrete shall be considered as 2% in
concrete with 20mm MSA, 1% in concrete with 40mm MSA and
3% in concrete with 10mm MSA. These air contents of concrete
are for non-air-entrained concrete, and is always entrapped in
fully compacted concrete.Estimation of Coarse and Fine Aggregate Content in Concrete.
With the quantities of cement, flyash, water, and
superplasticizer estimated, and the air content assumed, the total
absolute volume occupied by these materials in a cubic metre of
concrete is calculated. The remaining volume in a cubic metre of
concrete will be occupied by the coarse and fine aggregates.
The volume of coarse aggregate will depend on MSA, grading
of fine aggregate and the workability of concrete. For a normal
workability of say 50-60mm slump, for 20mm MSA (crushed
rock) and for fine aggregate of medium grading (say of zone
II or zone III as per Table 4 of IS: 383), the quantity of coarse
aggregate will be about 62-64%), of total aggregate by absolute
volume. If sand is of coarse variety (say of grading Zone I as
per Table 4 of IS: 383), the volume of coarse aggregate will be
around 60% of total aggregate by absolute volume. For 40mm
maximum size of crushed rock coarse aggregate, the above
mentioned volume of coarse aggregate will be higher by 0.09 m3
approximately, for different grading zones of fine aggregate.
For a high workability concrete mix of say 100-120 mm
slump, the volume of crushed rock coarse aggregate will be 10%
less than the values mentioned above for 20mm and 40mm MSA
in concrete.Accelerated Strength Testing of Concrete
The boiling water method of accelerated curing can be
employed to estimate, the 28-day compressive strength of
concrete in 28½ hours. The method is quite accurate and
explained in IS: 901310. Using this method 9 concrete cubes
from the three concrete mixes (with a variation of +10% in W/
(C+F) ratio cast, can be kept under boiling water for 3½ hours,
after the concrete cubes (in the mould) are normal-cured for
23 hours. The concrete moulds need to be covered by cover
plates and screwed, before placing them in boiling water. After
removing from boiling water, the concrete cubes are cooled for
2 hours, demoulded and tested. The derived equation from the
latest data of the Central Road Research Institute in IRC:8511 for
PPC (with 25-35% flyash) is :
R28 = 1.85 Ra N/ mm2
where, R28 = 28-day compressive strength of normal cured
concrete, N/ mm2 and
Ra = Accelerated compressive strength of concrete, N/ mm2
The W/(C+F) ratios Vs estimated average 28-day
compressive strength of concrete for the 3 mixes are next
plotted, and a correlation can be obtained, from which the W/
(C+F) ratio required for the target 28-day compressive strength
of concrete can be estimated. The concrete mix proportions for
this W/(C+F) ratio are then calculated, and can be recommended
for the field trial.
Typical worked out example on M25 grade of
flyash concrete of workability 100-120mm
slump, for reinforced concrete construction.
Degree of Quality Control : Good
Exposure condition: Moderate (Ref. IS: 456). Quantity of
flyash to be used: 25% by weight of (Cement + Flyash).
Test Data on Materials:
Cement: OPC, 43 grade
Maximum Size of coarse aggregate (crushed rock): 20mm
Grading of fine aggregate: Zone II (as per Table 4 of IS:383)
Combined grading of coarse aggregate (Two fractions: 10-
20 mm size & 10-4.75mm size combined in the proportions
60:40, satisfying the combined grading requirement of Table2 of
IS: 383. sp.gr of coarse aggregate: 2.74, water absorption: 0.5%.
sp.gr of fine aggregate: 2.62, water absorption: 1.0%
Condition of coarse aggregate: Dry
Condition of fine aggregate: wet (total moisture : 6%)
Quality of flyash: Good (Properties : Blaine’s Fineness = 325
m2/kg, lime reactivity = 4.6 N/ mm2), satisfying the requirement
of IS: 3812 (Part 1), sp.gr=2.20.
Superplasticizer (Naphthalene- based), sp.gr.=1.1Steps of Mix Design
(i) Target 28-day compressive strength of concrete = 25 + 1.64
x 4 = 31.6 N/ mm2
(ii) W/C+F ratio selected from Fig. 1 = 0.44
(iii) With the superplasticizer and flyash in hand, by trial, it was
observed that with 1% superplasticizer [by weight of (C+F)],
with 25% flyash, the desired workability of concrete of
100mm slump was achieved with 170l of water per cubic
metre of concrete.
(iv) The (cement + flyash) content of concrete = 170/0.44 = 386
kg/ m3 of concrete
OPC = 290 kg, Flyash (25%) = 96 kg.
The (C+F) content and W/(C+F) ratio satisfied the
requirements of minimum cementitious material content
(300 kg) and maximum W/cementitious materials ratio (of
0.50) as per Table 1, i.e. durability requirement for reinforced
concrete under ‘moderate’, exposure condition.
(v) Superplasticizer [1% of (C+F)] = 3.86 kg.
(vi) Volume of coarse aggregate (20mm MSA), using a fine
aggregate of grading zone II for high workability concrete
reducing by 10% =0.62 m3 x 0.90 = 0.56 m3 / m3 of total
aggregate by absolute volume.
(vii) Volume of fine aggregate = 1-0.56 = 0.44 m3 / m3 of total
aggregate by absolute volume.
(viii) Absolute volume of materials + (air content) except that of
aggregates :
a) Absolute volume of OPC = 290/3150 = 0.0920 m3
(assuming sp.gr of OPC = 3.15)
b) Absolute volume of flyash (sp.gr=2.2) = 96/2200 =
0.0436 m3
c) Absolute volume of water = 170/1000 = 0.1700 m3
d) Absolute volume of superplasticizer = 3.86/1100 =
0.0035 m3
e) Air content of concrete with 20mm MSA (2%)=0.0200 m3
Total = 0.329 m3
(ix) Absolute volume of (Coarse + fine aggregate) = 1- 0.329 =
0.671 m3
(x) Quantity of Coarse aggregate (S.S.D.)
(with sp.gr = 2.74) = 0.671 x 0.56 x 2740 = 1030 Kg.
(xi) Quantity of Fine aggregate (S.D.D.) (with sp.gr = 2.62) =
0.67 x 0.44 x 2620 = 772 Kg.
(xii)Quantity of Coarse aggregate (dry)
(capable of absorbing 0.5% moisture) = 1030 /1.005 =
1025 Kg.
(xiii) Quantity of Fine aggregate (wet) with 6% total moisture
= quantity of dry fine aggregate (which can absorb 1%
moisture) x 1.06 = 772/1.01 x 1.06 = 810 Kg
The coarse aggregate is dry,and the fine aggregate is wet.
Therefore, (1030 - 1025) ie 5 kg water is to be added for
the absorption of coarse aggregate, and (810-772) i.e. 38
kg. of surface water of fine aggregate is to be reduced from
the mixing water.
(xiv) Therefore, finally, mixing water content = 170+5-38=137 kg.
Quantities of all ingredients/ m3 of concrete :
OPC (43 grade) = 290 kg.
Flyash (25%) = 96 kg.
Superplasticizer (1%) = 3.86 kg.
The quantity of coarse aggregate (dry) = 1025 Kg.

10-20mm size (60%) = 615 Kg.
10-4.5mm size (40%) = 410 Kg.
Fine aggregate (wet) = 810 Kg.
Mixing water = 137 Kg.Note
1. The maximum cement content or cementitious material
content is to be reduced by 30 kg/ m3 for 40mm MSA, and is
to be increased by 40kg/ m3 for 10mm MSA.
2. The different exposure conditions are explained in IS 456.
References
1. IS 3812 (Part 1) Indian Standard Specification for pulverized
fuel ash, for use as pozzolana in cement, cement mortar and
concrete. Bureau of Indian Standards, New Delhi.
2. IS 383 Indian Standard specification for coarse and fine
aggregates from natural sources for concrete. Bureau of
Indian Standards, New Delhi.
3. IS 10262 Indian Standard guidelines for concrete mix
proportioning. Bureau of Indian Standards, New Delhi.
4. Teychenne, D.C., Nicholls, J.C., Franklin, R.E. and Hobbs,
D.W. Design of normal concrete mixes, The Department of
Environment, London.
5. BS 5328: Part 1. Guide to specifying concrete. British
Standards Institution, London.
6. Smith, I.A. The design of flyash concrete. Journal of Institution
of civil Engineers, Vol. 36, 1967, London.
7. ACI 211.4 R Guide for selecting proportions for highstrength
concrete with Portland cement and flyash. American
Concrete Institute.
8. IS: 456, Code of practice for plain and reinforced concrete.
Bureau of Indian Standards, New Delhi.
9. Baker, W.F. Burj Khalifa : A new paradigm. The Indian
Concrete Journal, Vol. 85, No. 7, July 2011, pp 8-13.
10. IS 9013 Method of making, curing and determining
compressive strength of accelerated cured concrete
test specimens, Bureau of Indian Standards,
New Delhi.
11. IRC: 85 Recommended practice for accelerated strength
testing and evaluation of concrete. Indian Roads Congress,
New Delhi.

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